How to unlock a MetroPCS Samsung Exhibit T599N for AT&T Service ?

I have a problem, I bought a MetroPCS Samsung exhibit phone off of an online store, since I use it only via WI-FI, it didn't really matter what carrier it was from. Now I got a contract and a new phone, and want to give the metro pcs phone to my father who has a contract with AT&T and his phone is about crappy and he needs a new one. I called MetroPCS and asked if they could unlock it, and of course they said,they couldn't. I don't really know anything about any of the companies that offer those unlock codes. I simply need to find a way of being able to give this phone to my father so he can use it with his existing at&t contract (sim card operated).
I would appreciate it if someone could let me know, if it is possible to get unlocked and how much it would cost.